With the spread of computer systems, mainly personal computers, there has been used a device for inputting new information or giving various instructions to a computer system by pointing at a position on a display screen of a display device on which information is displayed by the computer system, with an object such as a finger or a pointer. In order to perform an input operation with respect to the information displayed on the display screen of the display device of a personal computer or the like by a touching method, it is necessary to detect the contact position (indicated position) on the display screen with high accuracy.
As the touch panel devices for detecting the contact position of an object, a device using a resistance film and a device using ultrasonic waves have been well known. The former device using a resistance film detects a change in the resistance of the resistance film caused by the contact of the object with the resistance film. This device has the advantage of low power consumption, but has problems in the response time, detection performance and durability.
The latter device using ultrasonic waves propagates surface acoustic waves (SAWs) across a non-piezoelectric substrate and detects attenuation of the surface acoustic waves which is caused by contact of an object with the non-piezoelectric substrate. There has been proposed a touch panel device using an element composed of an IDT (Inter Digital Transducer) and a piezoelectric thin film as an excitation element for exciting a surface acoustic wave and a receiving element for receiving the propagated surface acoustic wave.
In this touch panel device, a plurality of excitation elements, each composed of the IDT and the piezoelectric thin film, are disposed on one end of the non-piezoelectric substrate and a plurality of receiving elements, each composed of the IDT and the piezoelectric thin film, are disposed on the other end of the non-piezoelectric substrate to face the excitation elements, respectively. An electric signal is inputted to each excitation element to excite a surface acoustic wave and propagate the surface acoustic wave across the non-piezoelectric substrate, and the propagated surface acoustic wave is received by the receiving element. Then, when an object comes into contact with the propagation paths of surface acoustic waves on the non-piezoelectric substrate, the surface acoustic waves attenuate. Accordingly, by detecting whether or not there is attenuation in the level of the received signals of the receiving elements, it is possible to detect whether or not contact was made.
In a conventional touch device, the IDTs of all the excitation elements and receiving elements have the same center frequency, and therefore contact of an object with the propagation path between a receiving element and the facing excitation element is detected by switching the IDTs to be connected to excitation/receiving circuits and detecting which receiving element's received signal attenuated.